In making color filter arrays, separate layers of different colors must be formed. Frequently these layers are formed using dye as the colorant material which is imbibed into dye receiving layers which must be of a controlled thickness, and a precise amount of dye must be used to achieve the appropriate color. See, for example, commonly assigned U.S. Pat. No. 4,764,670 to Pace et al and U.S. Pat. No. 4,876,167 to Snow et al and U.S. Pat. No. 5,576,265 to DeBoer et al which describe such a process. An additional problem associated with this process is that the dye receiving layers swell upon the introduction of dyes, limiting the smallest dimension which can be attainable for fabrication of very small filter elements. Another problem with this process is that dyes are susceptible to fading on exposure to light. In addition, the dyes spread within the dye receiving layer, resulting in loss in edge sharpness of the filter elements.
Turning to FIG. 1, which shows a method for making coatings from evaporated organic colorants. A substrate 102 is positioned adjacent to an aperture mask 104. The aperture mask provides an aperture over a portion of the substrate. An organic colorant which is to provide the coating is placed into a source boat 100, which is heated by passing an electric current through it. Alternatively, the boat may be heated by the application of radiant heating from a suitably placed heat source. Upon being heated under reduced pressure, the colorant vaporizes and travels from the source, impinging on mask 105. The portion of colorant vapor which passes through the opening in mask 105 travels along the lines 103, and between those lines, depositing on the substrate 102 and mask 104.
There are a number of problems associated with this technique which involves depositing layers in a partial vacuum and is frequently referred to in the art as physical vapor deposition (PVD). In certain cases, it is difficult to control the thickness and uniformity of the colorant disposed over the pixels. The process of vacuum deposition of the colorant typically requires the use of an appropriate placement of sources or masks or moving substrate fixtures to produce a coating which is uniform. However, the colorant material may deposit on the mask and vacuum fixtures to such a degree that it flakes off, creating undesirable contamination and waste of the colorant, and requiring frequent clean-up. In addition, the moving fixtures may generate undesirable particulate materials which may cause contamination of the substrate.
Some other problems in making color filter arrays by the PVD process are the need to use a large source-to-substrate spacing which requires large chambers and large pumps to reach a sufficient vacuum, and the need for masks which cause low-material utilization and build-up on the mask with the concomitant contamination problems. Very specific off-axis source location relative to the substrate, which is sometimes needed for uniform coating, causes very poor material utilization. Still further, source replenishment problems exist for coating multiple substrates in one pump-down. In addition, when multiple layers are deposited, the process needs to be carefully monitored for the thickness of layers in the multiple colorant coatings in multiple cycles.